1. Field of the Invention
This invention is in the field of clinical assay techniques.
2. Description of the Prior Art
Lipoproteins are complex particles consisting of protein and lipid which are found in the circulatory system. One of their functions is to carry water insoluble substances such as cholesterol and cholesterol esters for eventual cellular utilization. While all cells require cholesterol for growth, excess accumulation of cholesterol by cells is known to lead to certain diseases including atherosclerosis.
It is known that the amount of total serum cholesterol can be correlated with the incidence of atherosclerosis. However, there are a variety of classes of lipoproteins in serum which can be classified by their density. These classes include very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL). All of these lipoprotein classes contain varying amounts of cholesterol, and a total serum cholesterol determination is a complex average of the amount that each lipoprotein class contributes to the total lipoprotein population of the serum.
It has long been suspected that specific lipoprotein classes were more closely associated with the progression of heart disease, including atherosclerosis. In fact, more recent studies have implicated LDL as the class of lipoproteins responsible for the accumulation of cholesterol in cells whereas HDL has been shown to be important in the removal of excess cholesterol from cells. Additionally, the correlation of atherosclerosis and the levels of LDL cholesterol is much higher than a similar correlation between atherosclerosis and total serum cholesterol levels. Conversely, there seems to be a negative correlation of atherosclerosis and HDL cholesterol levels. See, Gofman, J. W., Jones, H. B., Lindgren, F. T., Lyon, T. P., Elliot, H. A., and Strisower, B., "Blood Lipids and Human Atherosclerosis," Circulation, 2:161-178 (1950); Barr, D. P., Russ, E. M., and Eder, H. A., "Protein-Lipid Relationships in Human Plasma, II, In Atherosclerosis and Related Conditions," Am. J. Med. 11:480-493 (1951); Nikkila, E., "Studies on Lipid Protein Relationships in Normal and Pathological Sera and Effect of Heparin on Serum Lipoproteins," Scand. J. Clin. Lab. Invest. Supplement, 5;1-101 (1952); Jencks, W. P., Hyatt, M. R., Jetton, M. R., Mattingly, T. W., and Durrum, E. L., "A Study of Serum Lipoproteins in Normal and Atherosclerotic Patients by Paper Electrophoretic Techniques," J. Clin. Invest., 35;980-990 (1956), and Miller, G. J. and Miller, N. E., "Plasma-High-Density Lipoprotein Concentration and Development of Ischemic Heart Disease" (technical note), Lancet, 1, (7897)16-19 (1975).
Despite the desirability of isolating LDL cholesterol levels in blood plasma from other soluble cholesterols, a technique suitable for use in clinical laboratories has not heretofore existed. The method most often used relies upon the interaction of heparin in the presence of calcium to precipitate both LDL and VLDL. See Burstein, M. and Scholanick, H. R., Adv. Lipid Res., 11, 67 (1973). To separate the LDL and VLDL fractions, ultracentrifugation techniques, which are time consuming and expensive, have to be employed.
Thus, a long existing need has existed for a simple, inexpensive, quantitative methodology to determine LDL cholesterol levels in blood plasma so that patients can be given a better assessment of their potential cardiovascular risk than that provided by presently used total serum cholesterol level assays.